Unless they are football tickets (where a specific offence exists under the Criminal Justice & Public Order Act) I cannot see that any criminal laws have been contravened and so this would be a civil matter, probably heard in the County Court. The company would have to take action for breach of contract, implying that in buying the tickets you entered into a contract with them and so were bound by their conditions (which probably include a �no resale� clause). They would have to go on to demonstrate that they had incurred losses as a result of your action and to quantify those losses.
However, what you have stated about the legality of selling tickets is, as far as I can recall, quite correct. I believe the conditions imposed by event organisers have been ruled unfair and so unenforceable. A couple of years ago there was a flurry of government activity and the views of all sides were examined. This actually resulted in nothing being done as far as I can recall.
It is interesting that the primary sellers of these commodities seek to control their resale, but are not against resale per se. They are quite happy for their approved �partners� to sell their wares, sometimes at a hefty mark up, but do not want to see Joe Public making a penny or two by doing likewise. Wimbledon Tennis, for example, imposes stringent conditions on members of the public who have been fortunate enough to secure tickets via their annual ballot, buy �Debenture Holders� who but seats for each day of the tournament for ten or fifteen years in advance, face no such restriction. They, along with many other event organisers, are also quite happy to sell tickets to corporate hospitality providers who, of course, simply sell them on.
I suggest you reply to the solicitors asking them on what basis they intend to take action against you.